fuse,fuse3: Avoid unnecessary build warnings

Do not set FUSERMOUNT_DIR since we already set it though
NIX_CFLAGS_COMPILE.

Setting it twice results in a lot of warnings like this one:

<command-line>:0:0: warning: "FUSERMOUNT_DIR" redefined

which makes the build look potentially broken.
This doesn't affect the result but fuse3 will now build without any
warnings and building fuse yields less warnings.